The Effect of Blade Geometry on Blade Stall Characteristics

The effect of rotation has been investigated with emphasis on the impact of blade geometry on the “correction factor” in stall models. The data used came from field tests and wind tunnel experiments performed by the National Renewable Energy Laboratory and were restricted to the steady-state nonyawed conditions. Three blade layouts were available; a blade with constant chord without twist (phase II), a blade with constant chord and twist (phases III and IV), and a tapered blade with twist (phase VI). Effects due to twist and taper were determined from comparison of$cn$between the different blade layouts. The formulation of the stall model was rewritten so that the measured$cn$values could be used without reference to 2D airfoil performance. This enabled a direct comparison of the normal force characteristics between the four blade stations of the selected blade configurations. In particular, the correction term$f$used in stall models for rotational effects was analyzed. The comparison between the test results with a straight and a twisted blade showed that a relation for twist$+$pitch is required in$f$⁠. In addition, a dependency of$f$on the angle-of-attack was identified in the measurements and it is recommended that this dependency be incorporated in the stall models.